It is required that a coating material for top coating of automobiles is excellent in curability at low temperature and storage stability and causes less surface bursting during the formation of a coating film, and is also capable of forming a coating film which is excellent in hardness, toughness, crosslinking density, acid resistance, thermal yellowing resistance, weatherability, water resistance, solvent resistance and scratch resistance. The term “surface bursting” means a phenomenon in which holes or depressions are formed due to the coating material repelled during the formation of a coating film.
As a main component of the coating material for top coating of automobiles, vinyl-based polymers have hitherto been used. Among these vinyl-based polymers, an acryl-melamine-based resin has widely been adopted because a coating film having excellent weatherability and decorative properties. However, a melamine resin contained as a curing agent in the acryl-melamine-based resin is insufficient in acid resistance. Therefore, when the resulting coating film is exposed to acid rain, the coating film is stained with rain and thus the appearance may become inferior. Therefore, various curing systems which utilize the crosslinking reaction of an acid group with an epoxy group have proposed as curing systems to be replaced by the melamine resin, for example, Japanese Unexamined Patent Application, First Publication No. Hei 2-45577 and Japanese Unexamined Patent Application, First Publication No. Hei 3-287650 propose the curing systems.
On the other hand, with the recent increase of an interest in an environmental problem, it has become important to decrease the amount of the solvent in the coating material, that is, to meet the “high solid-requirement”. However, a high solid coating material may cause problems such as increase in solution viscosity and poor coating workability.
A method which meets the high solid-requirement and also improves the workability (i.e. decrease in viscosity) of the coating material includes, for example, a decrease in molecular weight of the vinyl-based polymer. However, when the amount of a polymerization initiator used in the preparation is increased so as to decrease the molecular weight of the vinyl-based polymer, properties of the coating film such as acid resistance, weatherability and scratch resistance may be lowered. On the other hand, a method for decreasing the molecular weight of the vinyl-based copolymer without increasing the amount of the polymerization initiator includes, for example, the use in combination with a chain transfer agent such as mercaptanes. However, the chain transfer agent may cause deterioration of weatherability of the coating film.
By the way, a large amount of the unreacted monomer may be remained in the low-molecular weight vinyl-based polymer. The small amount of the unreacted monomer is preferable because it may cause surface bursting and foaming on the surface of the coating film during the use of the vinyl-based polymer.
In Japanese Unexamined Patent Application, First Publication No. Sho 59-6207, a method of preparing a low-molecular weight vinyl-based polymer with less unreacted monomer content is disclosed. According to this method, the vinyl-based monomer is polymerized at high temperature using a continuous tank reactor and then the unreacted monomer is removed by volatilization.
However, the method described in Japanese Unexamined Patent Application, First Publication No. Sho 59-6207 has such a problem that a large quantity of heat is required so as to volatilize the unreacted monomer, thus resulting in high cost. Even if the unreacted monomer is removed by the other method, a removing apparatus is required in the removing step and also the removing time is required, thus resulting in low productivity. Therefore, it has been required to develop a process which does not require the step of removing the unreacted monomer.
To decrease the amount of the unreacted monomer, polymerization may be carried out until the unreacted monomer substantially disappears, that is, the rate of polymerization reaches about 100%. However, it was difficult to control the rate of polymerization to 100% using a continuous tank reactor.
To prepare a vinyl-based polymer which exhibits a high rate of polymerization and also has a low molecular weight, it may be effective to raise the polymerization temperature and increases the retention time. If the polymerization temperature is too high, it exceeds so-called ceiling temperature and the depolymerization reaction as a reverse reaction dominantly occurs, and thus high temperature exerts an adverse effect on high rate of polymerization and the amount of by-products may increase.
It has hitherto been difficult to efficiently prepare the vinyl-based polymer, which exhibits a high rate of polymerization and also has a low molecular weight, only by raising the reaction temperature and increasing the polymerization time.